1. Field Of The Invention
The present invention relates to electronic circuitry. More particularly, the present invention relates to circuitry for programming an array of anti-fuses, and for preventing false programming of anti-fuses in an array of anti-fuses.
2. The Prior Art
The anti-fuse structure is similar to a capacitor; it contains an insulating material sandwiched between two conductors. For example, see application Ser. No. 861,519, Filed May 9, 1986, now U.S. Pat. No. 4,823,181 to Mohsen et al., assigned to the same assignee as the present invention. An anti-fuse may be programmed by applying a high voltage across the insulator until it ruptures, thereby shorting the two conductors together. The resistance of the ruptured anti-fuse is decreased by passing high current through it for an extended period of time. The period of voltage stressing is often called the stress time, and the period during which high current is allowed to flow is often called the soak time.
Anti-fuses may be used as a Programmable Read Only Memory (PROM) cell elements and may be placed in a PROM array. Some MOS anti-fuse memory PROM array circuits have reliability problems because the designs in which they reside have been implemented without taking into account the effects of substrate current and the parasitic bipolar devices in MOS circuits. The substrate current at issue is due to avalanching in the pinch off region of the MOS device. The parasitic bipolar device is composed of two or more like polarity diffusions separated by material of opposite polarity. For example, the drain and source diffusions of an MOS device are potentially the collector and emitter of a parasitic bipolar device whose base is either the substrate or the well in the substrate containing those diffusions.
If the substrate current is large enough, it can forward bias the base emitter junction of the parasitic bipolar device, thus turning it on. This parasitic device multiplies the substrate current by the beta of the bipolar transistor. A bata greater than 100 is possible, so a small substrate current can produce a large bipolar current.
A bipolar device may have multiple collectors. Therefore, one forward biased junction can draw collector current through numerous nearby junctions. These multiple collectors may cause reliability problems. For example, when one of a group of adjacent anti-fuse memory cells is being programed, if the parasitic bipolar device associated with one memory cell is turned on during the soak time, a current path via one of these parasitic multiple collectors may cause sufficient stress voltage across the adjacent unselected anti-fuses to rupture one or more of them and thus falsely program one or more of them.